Increasing fuel economy standards have motivated automakers to reduce vehicle mass with multi-material bodies-in-white. One joining technology particularly well suited for one-sided multi-material joining is Flow Drill Screwdriving (FDS), a process that joins two materials together by friction drilling a pilot hole and then forming threads and tightening the fastener to a given torque, all using a self-tapping screw fastener that passes through the stack-up workpiece. This results in a mechanical threaded joint and allows for dissimilar lightweight materials to be incorporated in the joined structures, which can provide cost benefits in both formation and use. For instance, in the specific application of transportation, the ability to quickly and safely join two dissimilar lightweight materials can reduce vehicle costs as well as fuel consumption.
As illustrated in FIG. 1, the FDS process is a dry process that includes six steps classified as: heating (1), penetration (2), extrusion forming (3), thread forming (4), screwdriving (5), and final torqueing (6). FDS induces thermal softening of the stack-up materials through frictional heating by use of a conical-tipped fastener that forms an elongated region of the stack-up materials and allows for threading of the materials (FIG. 2). FDS requires time for the heat to build, having a process time of 1-2 seconds (exclusive of the fastener driving element finding process), with the torque to thread-form workpiece materials limited to 8.3 N-m for M5 fasteners and 12 N-m for heavier M6 fasteners (FIG. 3). Beneficially, by use of FDS, the fastener is removable (which is ideal for repairability), and the same fastener/machine combination can be used regardless of material type or thickness, only requiring a change in parameters for different material characteristics. FDS has thus become the leading alternative to self-piercing riveting (SPR) when the joining back side is not accessible.
While FDS has become the leader in one-sided multi-material joining, issues still exist. For instance, stack-ups are generally limited to a thickness of about 7 mm, and materials in transportation applications (among others) are transitioning from thin steel sheets to aluminum extrusions as thick as 5 mm. In addition, one-sided joining of higher strength steels is not possible, with FDS being limited to low strength steel or aluminum. FDS is also a slow process compared with other joining technologies (e.g. resistance spot welding, Rivtac, friction element welding, SPR), with 50% of total process time coming from the first three forming steps (heating, penetrating, extrusion forming).
A significant process variable of FDS is the axial force, Faxial, applied to the fastener to penetrate the workpiece. As the axial force increases, process time decreases, which is sought after but not at the expense of an increased installation torque or base material deformation. A lowered axial force allows for a higher heat generation due to the extended contact time between the tool and the workpiece, but this increases process time. A higher axial load decreases process time resulting in lower part temperature and a higher installation torque. Installation torque is limited by the torsional strength of the fastener. For instance, the installation torque limit on FDS fasteners is 8.3 N-m (M5) and 12 N-m (M6) due to the standardized values of self-tapping screws.
Attempts have been made to improve FDS processing techniques. For instance, the effect of thermal assistance on the FDS process has been examined using a preheating conduction ring. Samples were preheated to various temperatures and a 20% reduction in installation torque and 52% reduction in process time were observed on those samples preheated to 247° C. Unfortunately, using a conduction ring to pre-heat requires an extended time for the material to reach the desired temperature.
What is needed in the art is FDS with improved processing. For instance, methods and devices that can raise the temperature of the materials prior to and/or during joining could be beneficial by providing decreased installation torque and decreased processing time, which could also provide for FDS joining of thicker workpieces.